Seeding with crystals

C,H,CH,- —C-NH,j The compound separates in either, sometimes as both, of two dimorphic forms, m.p. 150° and 175° respectively. The former may be converted into the higher m.p. form by dissolving It in alcohol and seeding with crystals of the form, m.p. 175° the low m.p. form when warmed to 175° gives, after sohdification, a m.p. of 175°. Both dimorphic forms give identical derivatives with carboxyUc acids and sulphonic acids (see Sections 111,85 and IV,33). 

Several techniques are usually employed to induce crystallization from saturated solutions of organic solids. The introduction of seed crystals will invariably work, although with new compounds such crystals are not available. Seeding with crystals of a... 

This oil Is kept supercooled at the ambient temperature. Crystallization may be obtained by cooling or by seeding with crystals of the product. The melting point is 34°C (instantaneous on the Maquenne block). 

In the Bayer process, the bauxite is leached with hot sodium hydroxide, thus forming a solution of sodium aluminate. After purification this solution is seeded with crystals of gibbsite and cooled. The process steps are summarised in Eqs. (1) and (2) ... 

According to Kaufman [91] spontaneous explosion can also take place during the growth of a-lead azide crystals, e.g. when a supersaturated solution of lead azide in ammonium acetate is seeded with crystals of the a-form. Spontaneous explosions have also been observed with mercuric azide and in some cases with cadmium azide. 

The solution is seeded with crystals of vincaleukoblastine sulfate and is chilled for about 24 hours. Vincaleukoblastine sulfate, if present, precipitates during this period and can be separated by filtration. Vincaleukoblastine sulfate melts at about 284° to 285°C. 

In the case of metastable phases the system may remain indefinitely in equilibrium without the appearance of a new phase. On the other hand if nuclei of a new, more stable, phase are introduced the system changes over into this phase. This is, for example, what happens in the case of supercooled liquids which can remain unchanged indefinitely but crystallize when seeded with crystals of the solid phase. 

The melting point is occasionally used as a criterion of the purity of nitroglycerine. Nitroglycerine supercools very readily and must he seeded with crystals of the solid substance. 

The requirements for homochirally pure a-amino acids have not ruled out any of these general synthetic methods (which all give racemic products), since resolution of DL-a-amino acids and their derivatives is a simple, albeit time-consuming, solution to this need. Classical methods for resolution include physical separation of the DL-amino acids themselves (by chromatography on a chiral phase e.g. resolution of DL-tryptophan over cellulose, see Section 4.15), fractional crystallisation of certain racemates or supersaturated solutions (through seeding with crystals of one enan-... 

Aluminum hydroxide crystals are obtained from the supernatant sodium aluminate solution after a final filtration by ensuring a Na20 AI2O3 ratio of 1.5-1.8 1, by controlling the solution temperature to about 60°C, and by seeding with crystals from an earlier crystallization. Even with seeding the crystallization process requires 2-3 days. However, sufficiently large crystals are obtained by the reverse of the solution process (Eq. 12.4) to enable product recovery by filtration. 

Seed with crystals of the appropriate amount and size. 

Crystals from ale or dil HC1, mp 172-174°, Metastable form, mp 146-148, Can be converted to the higher melting form by dissolving in alcohol and seeding with crystals, mp 172-174°. 

It is known since 1975 [10] that crystal chemical reactions can be used in absolute asymmetric synthesis When achiral molecules crystallize in chiral space groups and the reaction of the crystals leads to chiral products, one speaks justly and correctly of absolute asymmetric synthesis. For this, the use of chiral agents and thus also the crystal-selecting human hand must be dispensed with. However, if autoseeding does indeed occur in a system, manipulations by seeding with crystals of a desired chirality must not detract from that term still. Most absolute asymmetric syntheses have been performed photochemi-cally. Some short reviews have appeared. [11]... 

Effect on the solubility of sulfamethoxydiazine Form 11 by seeding with crystals of Form 111. Shown are the dissolution profiles of Form 11 (- -), Form 111 ( -), and Form 11 seeded with Form 111 after 20 minutes elapsed time (- -). (The plots were adapted from data originally presented in Ref. 31.)... 

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